The invention relates to a multiple omnidirectional antenna employing a plurality of antenna sections designed for different operational frequencies and disposed along a common axis and utilizing multiple coaxial feed from one side only.
It is known, for the simultaneous reception and simultaneous emission of electromagnetic waves of different frequencies in the microwave range with specific polarizations, in which an omnidirectional diagram is formed in the horizontal plane, and a wide radiation diagram is formed in the vertical plane, to employ omnidirectional antennae which are disposed adjacent to one another and are selected for the particular polarization desired. However, with such an arrangement a mutual influence or reaction occurs between the individual antennae, thus producing interference which is unacceptable in most applications, and in particular in connection with secondary radar transponder devices which are intended to simultaneously effect a coding operation at a plurality of frequencies.
German OS No. 23 54 550 discloses an integrated double omnidirectional antenna with a double coaxial feed. The two antennae which operate at different frequencies are constructed along a double coaxial line structure in two sections, one disposed above the other, on a single axis, in which the lower and upper section can be selectively designed either as a unipole for vertical polarization or as coaxial line radiator, provided with a slot and a short-circuiting pin for horizontal polarization. This known integration is, in itself, sufficient to combine two omnidirectional antennae for more or less different frequencies, in such a way that their omnidirectional characteristics are not disturbed as a result of their disposition on a common axis.